Need Help Fast!!

Hi, I have a 1996 2 dr 4wd tahoe. After about 15 miles on the freeway the service engine soon light comes on. Took it to autozone and the computer said P1780? was showing up for a short circuit to the auto trans switch. took it to the dealer, they want 600 to take it out, and 600 to put it back in...and watever it costs to fix it. They said they have to take it to find the short. If it is a short circuit, why do they need to take the tranny out? Is something wierd going on here...Thanks. PS, the tranny shifts fine and everything else feels fine. Thanks again